Lecture 2. What are air pollutants: preliminary review
Objectives:
1. Air pollution definitions.
2. Air pollution classifications.
3. Major air pollutants: introduction.
Readings: Turco: p. 3-7, 111-112
1. Air pollution definitions.
Air pollution may be defined as any atmospheric condition in which
substances are present at concentrations high enough above their normal
ambient levels to produce a measurable effect on man, animals, vegetation,
or materials.
Substances mean any natural or anthropogenic (man-made) chemical compounds
capable of being airborne. They may exist in the atmosphere as gases, liquid drops, or
solid particles.
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Measurable effects on humans and environment due to:
‘indoor’ air pollutants (Lecture 15),
air toxics, radioactivity (Lecture 16)
urban photochemical smog (Lectures 19-21),
acid rain (Lectures 22-24),
visibility reduction (Lectures 25-26)
greenhouse warming (Lectures 33-34),
depletion of the ozone layer (Lectures 35-38),
climate forcing due to anthropogenic atmospheric aerosols (Lectures 39-41)
Figure 2.1 Example of air pollution effects.
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Figure. 2.2. The air pollution problem can be schematically presented as a system
consisting of three basic components:
Major emission sources:
Receptors:
1) transportation;
1) Humans
2) industrial and domestic fuel burning
2) Animals
3) industrial processes
3) Plans
4) Materials
Atmosphere
acs as a medium for
transport,
dispersion,
physical and
chemical transformations
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2. Air pollution classifications.
according to chemical composition:
1. Sulfur-containing compounds.
2. Nitrogen-containing compounds.
3. Carbon-containing compounds.
4. Halogen-containing compounds.
5. Toxic substances ( any of about)
6. Radiative compounds.
according to physical state:
1. Gaseous
2. Liquid (aqueous)
Particulate matter
3. Solid
according to the manner in which they reach the atmosphere:
1. Primary pollutants (those emitted directly from the sources)
2. Secondary pollutants (those formed in the atmosphere by chemical interactions among
primary pollutants and normal atmospheric conditions)
according to the space scales of their effects:
1. Local (or indoor)
2. Regional
3. Global
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•
Criteria air pollutants are six major pollutants defined by EPA (Environmental
Protection Agency) for which ambient air standards have been set to protect human
health and welfare.
Criteria pollutants (defined by EPA):
1. Ozone, O3.
2. Carbon monoxide, CO.
3. Sulfur dioxide, SO2.
4. Nitrogen oxides, NOx.
5. Lead, Pb.
6. Particulates, PM10.
3. Major air pollutants
Ozone as a pollutant.
Ozone, O3, is a gas,
•
At ground level, ozone is a hazard (‘bad’ ozone) - it is a major constituent of
photochemical smog. However, in the stratosphere, it serves to absorb some of the
potentially harmful UV radiation from the sun which is believed to cause skin cancer,
among other things (‘good’ ozone).
Sources: ozone is not emitted into the atmosphere; ozone is formed from the ozone
precursors, VOCs, and nitrogen oxides (will be discussed in several Lectures).
“Bad” ozone effects:
•
diverse effects on human health
•
ecological effects: damage vegetable and trees,
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Major sulfur-containing compounds :
Sulfur dioxide, SO2, is a colorless gas with a sharp odor, primary pollutant, has
anthropogenic (man-made) and natural sources.
Anthropogenic sources: industries burning sulfur-containing fossil fuels, ore smelters, oil
refineries.
•
Sulfur is present in many fuels (e.g., coal, crude oils) over a wide range of
concentrations. Combustion causes its oxidation to sulfur dioxide.
Natural sources: marine plankton, sea water, bacteria, plants, volcanic eruption.
SO2 effects:
•
At relatively high concentrations SO2 causes severe respiratory problems.
•
Sulfur dioxide is an acid precursor which is a source of acid rain produced when SO2
combines with water droplets to form sulfuric acid, H2SO4 (discussed in Lectures 2224).
•
Sulfur dioxide is an precursor of sulfate particulates (sulfates) which affect the
radiation balance of the atmosphere and can cause global cooling (discussed in
Lecture 39).
Major nitrogen-containing compounds
•
Nitrogen, N2, is a dominant gas of the atmosphere about 78% by volume
(discussed in Lecture 3)
•
NOx stands for an indeterminate mixture of nitric oxide, NO, and
nitrogen dioxide, NO2
Nitrogen oxides, NOx, are formed mainly from N2 and O2 during high-temperature
combustion of fuel in cars.
Anthropogenic sources: motor vehicles, biomass burning
Natural sources: bacteria, lightning, biomass burning
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NOx effects:
•
causes the reddish-brown haze in city air, which contributes to heart and lung
problems and may be carcinogenic
•
NOx is an acid precursor which is a source of acid rain produced when nitrogen
oxides combines with water to produce nitric acid, HNO3, and other acids (discussed
in Lectures 22-24).
•
Nitrogen oxides are the precursors of nitrate particulates (nitrates) which affect the
radiation balance of the atmosphere and can contribute to global cooling
•
Nitrogen oxides are major contributors to the formation of ground level ‘bad’ ozone.
Figure 2.3 SO2 and NOx emissions in US (by source)
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Major carbon-containing compounds
Carbon monoxide, CO, is a colorless odorless flammable gas, major pollutant of an
urban air, produced from incomplete combustion.
Anthropogenic sources: petrol engined motor cars, cigarette smoke, biomass burning
Natural sources: biomass burning
NOTE: CO is also produced by atmospheric oxidation of methane gas and other
hydrocarbons.
CO effects:
• CO is highly poisonous to humans and most animals: when inhaled, CO reduces the
ability of blood hemoglobin to attach oxygen.
NOTE: Don’t confuse carbon monoxide, CO, and carbon dioxide, CO2. Carbon dioxide
is a complete oxidation product of fuel combustion. Also, in the atmosphere, CO
oxidized to CO2.
Carbon dioxide, CO2, is a key greenhouse gas (discussed in Lectures 33-34)
Principal sources: fossil fuel combustion, deforestation, cement production.
•
USA is the single largest emitter of CO2 accounting for about 16% of the world total.
Hydrocarbons and volatile organic carbons (VOCs):
organic gases are those that contain both hydrogen and carbon, but may also contain
other atoms; hydrocarbons (HCs) are organic gases that contain only hydrogen and
carbon. Volatile organic compounds (VOCs) are non-methane hydrocarbons (NMHC)
and oxygenated hydrocarbons (which are hydrocarbons plus oxygenated functional
groups), (will be discussed in Lecture 8).
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Methane, CH4, is the most abundant hydrocarbon in the atmosphere, found in exhaust
gas from automobiles, biomass burning, agriculture activities (e.g., rice paddies).
Anthropogenic sources: indoor sources (e.g., formaldehyde emission), fossil fuel
combustion, evaporation of gasoline (e.g., petroleum refineries; during fueling of cars),
Natural sources: HCs produced from decomposition of organic matter; emitted by certain
types of plains (e.g., pine trees, creosote bushes)
Effects:
•
some HCs are indoor pollutants (e.g., formaldehyde discussed in Lecture 15)
•
some HCs and VOCs contribute to ozone-containing smog
Example: pine trees produce VOCs such terpenes responsible for the smell of pines,
unfortunately these pleasant VOCs contribute to ozone formation that harms trees.
Major halogen- containing compounds
Chlorofluorocarbons, CFCs, are artificial gases, used as the coolants in refrigerators
and air conditioners,; they are neither toxic nor flammable. The most abundant CFCs are
CFC-11 (or CFCl3), and CFC-12 (or CF2Cl2).
•
CFCs are artificial halocarbons, therefor they are not biodegradable. CFCs are not
water soluble, therefore they are not washed from the atmosphere by rain. In the
stratosphere, UV radiation destroy CFCs breaking them down to a few chemicals
(including atomic chlorine and atomic bromine which efficiently destroy ozone).
CFCs effects:
•
they are the key greenhouse compounds (discussed in Lecture 33)
•
they lead to reduction of stratospheric “good” ozone (discussed in Lectures 35-38)
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Metals as the pollutants.
Metals (such as lead, mercury, cadmium, chromium, nickel) found as impurities in fuels.
Anthropogenic sources: emitted by metal mining and processing facilities; motor vehicle.
Example: lead is a very useful metal, has been mined for thousand of years
Main effect: They are highly toxic (discussed in Lecture 16)
Particulate matter (aerosols) are solid or aqueous particles composed of one or
several chemicals and small enough to remain suspended in the air (discussed in several
Lectures).
Examples: dust, soot, smoke, sulfates, nitrates, asbestos, pesticides, bioaerosols (e.g.,
pollen, spores, bacterial cells, fragments of insects, etc.)
PM(10) are particles with diameter < 10 micrometer (µm).
Anthropogenic sources: various (biomass burning, gas to particle conversion; industrial
processes; agriculture’s activities)
Natural sources: various (sea-salt, dust storm, biomass burning, volcanic debris, gas to
particle conversion)
Effects:
•
diverse health effects (e.g., harmful to human respiratory system)
•
contribute to urban haze, cause visibility reduction (discussed in Lectures 25-26)
•
play a key role in the Earth’s radiative budget and global change (discussed in
Lectures 39-41)
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Figure 2.4 Primary pollutant emissions in the United States.
VOCs
15%
Particulates
5%
CO
48%
NOx
16%
SO2
16%
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